This invention relates to methods for, producing electrode active materials which can be used to formulate electrodes for use in electrochemical cells in batteries. More-particularly, the present invention relates to methods for the production of electrode active lithium metal phosphate materials. Even more particularly, the present invention relates to methods whereby electrode active materials having unique triclinic or olivine crystalline structures can be produced.
Lithium batteries have become a useful and desirable energy source in recent years. Generally speaking lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material have received acceptance in industry and commerce.
By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through a liquid electrolyte to the electrochemically active (electroactive) material of the cathode whereupon they release electrical energy to an external circuit.
It has recently been suggested to replace the lithium metal anode with an insertion anode, such as a lithium metal chalcogenide, lithium metal oxide, coke or graphite. These types of electrodes are typically used with lithium-containing insertion cathodes to form an electroactive couple in a cell. The resulting cells are not charged in an initial condition. Before this type of cell can be used to deliver electrochemical energy, it must be charged. In the charging operation, lithium is transferred from the lithium-containing cathode to the anode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it reinserts. Thus with each charge/discharge cycle, the lithium ions (Li+) are transported between the electrodes. Such rechargeable batteries, having no free metallic species, are called rechargeable ion batteries or rocking chair batteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Various materials have been suggested and employed as the cathode material in the aforementioned batteries. Preferred positive electrode active materials generally include LiCoO2, LiMn2O4, and LiNiO2. These materials are synthesized by a variety of synthesis modes which can generally be classified as xe2x80x9cwet method synthesisxe2x80x9d. Methods of making lithium compounds are described in U.S. Pat. No. 5,135,732 by Barbus, et al. and U.S. Pat. No. 4,246,253 by Hunter, and involve the formation of aqueous solutions as intermediate steps. Lithium compounds containing cobalt are relatively expensive to synthesize due to the intermediates required, while successful synthesis of lithium-nickel compounds is relatively complex and difficult. Lithium-manganese compounds, such as LiMn2O4, are generally more economical to synthesize than the preceding material and result in a relatively economical positive electrode.
Unfortunately all of the foregoing materials have inherent drawbacks when employed as electroactive materials in electrochemical cells. Cells employing each of the foregoing materials in the cathode experience significant loss of charge capacity over repeated charge/discharge cycles, commonly referred to as cycle fading. The initial capacity available (amp hours/gram) from materials, such as LiMn2O4, LiNiO2, and LiCoO2, is less than the theoretical capacity because significantly less than 1 atomic unit of lithium engages in the electrochemical reaction. This initial capacity value is significantly diminished during the first cycle of operation and diminishes even further on every successive cycle of operation. Thus for LiNiO2 and LiCoO2 only about 0.5 atomic units of lithium is reversibly cycled during cell operation.
Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Niagara et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing electrode material having acceptable capacity without the disadvantage of significant capacity loss when used in a cell.
In related applications, U.S. Ser. No. 09/204,944 and U.S. Ser. No. 09/559,861 which are currently pending before the United States Patent and Trademark Office, the inventors have disclosed novel lithium metal phosphate and lithium metal fluorophosphate materials which address concerns such as cycle fading and the like. However, there remains a long-felt and, as yet, unsatisfied need for providing an economical and reproducible synthesis method for such phosphate-containing materials which will provide good quality material in suitable yields.
This invention provides a method of making lithium metal phosphate compounds suitable for use as active materials in electrodes. In the method of the present invention, the various materials utilized are in particulate form and include at least one metal compound and at least one phosphate compound. These materials are present as solid particulate materials and are admixed in the presence of a reducing agent at a suitable reaction temperature in an appropriate non-oxidizing environment. The particulate metal, particulate phosphate and reducing agent remain in contact with one another for an interval and at a temperature sufficient to form a particulate metal phosphate reaction product. The resulting metal phosphate reaction product characteristically contains a metal ion derived from the particulate metal compound and a phosphate ion derived from the particulate phosphate compound.
The resulting metal phosphate reaction product is reacted with a source of lithium ions in a manner sufficient to form a lithium metal phosphate reaction product.
In more specific embodiments of the present invention, there is provided novel methods of making lithium-metal-fluorophosphate materials; new materials which, upon electrochemical interaction, release lithium ions, and are capable of reversibly cycling lithium ions. Such materials can be employed in various ways, including but not limited to, use in a rechargeable lithium battery which comprises an electrolyte; a first electrode having a compatible active material; and a second electrode comprising the novel lithium-metal-fluorophosphate materials. Lithium-metal-fluorophosphate materials produced by the process of the present invention can be represented by the nominal general formula LiM1xe2x88x92yMIyPO4F where 0xe2x89xa6yxe2x89xa61. Such compounds include LiMPO4F for y=0. Such compounds are also represented by Li1xe2x88x92xMPO4F and Li1xe2x88x92xM1xe2x88x92yMIyPO4F, where in an initial condition, xe2x80x9cxxe2x80x9d is essentially zero; and during cycling a quantity of xe2x80x9cxxe2x80x9d lithium is released where 0xe2x89xa6xxe2x89xa61. Correspondingly, M has more than one oxidation state in the lithium metal fluorophosphate compound, and more than one oxidation state above the ground state M0. The terms oxidation state and valence state are used in the art interchangeably.
Broadly construed, the method of making lithium metal phosphate materials of the present invention utilizes precursor materials in particulate or powder form. The terms powder, particle, and particulate are used interchangeably herein. Particulate starting materials include a phosphate compound, at least one metal compound, in intimate admixture with one another and in intimate contact with a reducing agent. The reducing agent, optionally, can be a metal in its elemental state. The admixture and reducing agent of the starting materials is heated under conditions which do not support oxidation. The reaction temperature and interval are generally defined as those sufficient to form a reaction product comprising the metal and the phosphate. The starting material may comprise more than one metal compound provided that at least one of the metal compounds employed is a transition metal compound.
The resulting metal phosphate compound is mixed with a lithium compound. The resulting mixture is then heated at a sufficient temperature and for a sufficient time to form a reaction product comprising the metal phosphate and the lithium having the nominal general formula: LiM1xe2x88x92yMIyPO4 where 0xe2x89xa6yxe2x89xa61. Such compounds include LiMPO4 for y=0.
In order to produce the lithium metal fluorophosphate material, the resulting metal phosphate produced as above can be admixed with a fluorine-containing lithium compound. The resulting admixture is then heated at a sufficient temperature and for a sufficient time to form a reaction product comprising the metal phosphate, the lithium and fluorine. It is also considered within the purview of this invention to utilize particulate metal phosphate materials derived from other synthetic methods in admixture with materials such as lithium fluoride to produce a lithium metal fluorophosphate.